The present invention relates to an FET balun transformer.
In recent years, the operating frequencies of communications devices have been further raised or broadened to transmit and receive data of even larger capacities. For these purposes, a balun transformer is needed to convert a single-ended signal into differential signals with a phase difference of 180 degrees. A passive balun transformer, which is a combination of coils, has been generally used for a radio frequency circuit. Currently, however, an FET balun transformer, which includes multiple field effect transistors (FETs), is often used to meet the demand of broadening the signal frequency range or integrating as many devices as possible on the same chip. As for a signal with a frequency exceeding 1 GHz, which belong to a microwave or quasi-microwave range, in particular, an FET balun transformer is regarded as effectively contributing to downsizing the device.
Furthermore, a high-performance balun transformer, which includes gallium arsenide (GaAs) Schottky gate field effect transistors (i.e., MESFETs) exhibiting excellent radio frequency characteristics and low distortion while consuming just a small amount of current, is also used widely.
Hereinafter, a known FET balun transformer will be described with reference to FIG. 6.
FIG. 6 illustrates a prior art FET balun transformer. As shown in FIG. 6, the balun transformer includes first, second and third FETs 31, 32 and 33. The drains of the first and second FETs 31 and 32 are connected to a positive power supply 36 by way of load resistors 34 and 35, respectively.
The gate of the first FET 31 is connected to an input terminal 37, while the gate of the second FET 32 is grounded. The sources of the first and second FETs 31 and 32 are connected in common to the drain of the third FET 33, which operates as constant current source. The source of the third FET 33 is connected to a negative power supply VSS of -1 V, for example, via a biasing resistor 38, while the gate thereof is connected to the negative power supply VSS directly. The drains of the first and second FETs 31 and 32 are connected to first and second output terminals 39 and 40, respectively.
The gate and source of the third FET 33 are both connected to the negative power supply VSS because of the following reasons. Since a MESFET uses a Schottky gate, the gate-source voltage should be negative. Accordingly, if the gate biases of the first and second FETs 31 and 32 are set at 0 V, then the gate and source of the third FET 33 should be less than 0 V.
Next, it will be described how the conventional FET balun transformer operates. A single-ended RF signal is received at the input terminal 37. In response, the current flowing through the first FET 31 changes. However, since the gate of the second FET 32 is grounded and a constant current flows through the third FET 33, the total amount of currents flowing through the load resistors 34 and 35 does not change but the drain potential of the third FET 33 changes.
Thus, so long as the single-ended signal received at the input terminal 37 is located in the linear region of the first FET 31, the drain voltage of the third FET 33 is equal to the source voltage of the first FET 31. As a result, signals with mutually inverted phases are output through the first and second output terminals 39 and 40.
The conventional FET balun transformer, however, needs not only the positive power supply 36 for supplying a positive potential to the drains of the first and second FETs 31 and 32, but also the negative power supply VSS to set the gate and source of the third FET 33 at a negative potential. A device with the negative power supply VSS is too complicated and too large to be applied to an IC.